1. Field of the Invention
The present invention pertains to compositions for use in the drilling and completion of subterranean wells. More specifically, the present invention relates to dispersants for spacer fluids, cement slurries and mixtures of drilling fluids and cement slurries.
2. Description of the Prior Art
Techniques are well known for drilling and completing wells, particularly gas and oil wells, which are drilled from the surface of the earth to a subterranean formation containing a fluid mineral which it is desired to recover. After the fluid-containing geologic formation is located by investigation, a borehole is drilled through the overlying layers of the earth's crust to the fluid-containing geologic formation to permit recovery of the fluid mineral contained therein.
The drilling fluids used during drilling of the borehole are generally classified on the basis of their principal component. When the principal ingredient is a liquid (water or oil) the term "mud" is applied to a suspension of solids in the liquid. The most common drilling fluids contain water as the principal component. These drilling fluids are known as water-base drilling fluids or "water muds".
Water-base drilling fluids vary widely in composition and properties and many have been classified based on their principal ingredients. Some common classifications of water-base drilling fluids are freshwater muds, low solid muds, spud muds, salt water muds, lime muds, gyp muds and CL-CLS muds.
In addition to water, drilling fluids often include any number of known additives which perform various functions in the drilling fluid system. Among the most common additives are materials which increase density such as barite and galena and viscosifiers or thickening agents such as clays (usually bentonite).
Furthermore, many water-base drilling fluids are formulated to contain one or more polymeric additives which serve to reduce the rate of filtrate loss, stabilize clays, flocculate drilled solids, increase carrying capacity, emulsify, lubricate and the like. Among the most commonly employed polymeric additives are starches, guar gum, xanthan gum, sodium carboxy-methylcellulose (CMC), hydroxyethylcellulose (HEC), carboxy methyl hydroxyethyl cellulose (CMHEC) and synthetic water dispersable polymers such as acrylics and alkylene-oxide polymers.
Also well known and important for their ability to reduce flow resistance and gel development in clay-water muds are materials which are broadly referred to in the drilling fluid industry as "dispersants". Materials conventionally used-as dispersants are classified as plant tannins, polyphosphates, lignitic materials, synthetic water dispersable polymers and lignosulfonates.
When the drilling of the borehole is completed, a casing is secured in position within the borehole to insure permanence of the borehole and to prevent entry into the well of a fluid from a formation other than the formation which is being tapped. The well casing is typically cemented in place by pumping a cement slurry downwardly through the casing. The cement slurry flows out of the open lower end of the casing at the well bottom and then upwardly around the casing in the annular space between the outer wall of the casing and the wall of the well borehole. Frequently, a spacer fluid is pumped downwardly through the casing ahead of the cement slurry to form a buffer between and prevent the contact of the drilling fluid and the cement slurry which are typically incompatible fluids.
The drilling process which produces the borehole will usually leave behind on the wall of the borehole produced, a drilling fluid filter cake of mud-like material. This material is a barrier to the formation of proper bonding by any cement composition employed to produce an impermeable bond between the casing and the well wall.
In an ideal cementing operation the drilling fluid filter cake would be removed from the well bore wall and replaced by the cement slurry to permit the formation of a solid layer of hardened, cured and bonded cement between the casing and the geologic formations through which the well borehole passes. It has been recognized in the art that removal of the drilling fluid filter cake is greatly enhanced by injecting the cement slurry, spacer fluid or the like into the well borehole under turbulent flow conditions,
Inducing turbulence by control of flow velocity alone requires a specific minimum velocity, which in turn is limited by the maximum pressure the wellbore can tolerate. Particularly, where the turbulence induced is sufficient to assure removal of the drilling fluid filter cake, additional pumping capacity and very high pressure levels are usually required. These required pressure levels, especially for deep wells, often exceed the pressure at which the subterranean formations break down, resulting in lost circulation. In addition, the required pressure level may exceed the capacity of the pumping equipment or the endurance of the well drilling and associated apparatus.
The pumping of cement slurries in turbulent flow at lower flow velocities than would be possible using conventional cement slurry compositions, has been accomplished in the prior art by adding to the said cement slurries a flow-property-improving and turbulence-inducing agent, which agent is also known as a dispersant.
In the cementing of oil and gas wells, rather than displacing and removing the drilling fluid during cement slurry placement, it has been proposed to convert the drilling fluid to a cementitious slurry for cementing casing or tubing in place or otherwise stabilizing or protecting the casing by sealing the formation in the vicinity of the borehole.
The conversion of drilling fluid or "mud" to a cement slurry, however, is not without some operational problems and undesirable compositional changes. For example, the addition of cementitious materials such as mixtures of lime, silica and alumina, or lime and magnesia, silica and alumina and iron oxide, or cement materials such as calcium sulphate and Portland cements to aqueous drilling fluids can substantially increase the viscosity of the fluid mixture and result in severe flocculation. Efforts to circulate such mixtures through a wellbore can result in a highly unsatisfactory circulation rate, plugging of the wellbore annulus, breakdown of the earth formation in the vicinity of the wellbore and a failure of the cement slurry to properly mix.
These problems can be overcome by the addition of a dispersant composition which disperses both the drilling fluid and the cement slurry mixture. However, conventional drilling fluid dispersants do not necessarily disperse cement slurries and conventional cement slurry dispersants do not necessarily disperse drilling fluids. In addition, conventional dispersants which do disperse both drilling fluids and cement slurries do not necessarily disperse mixtures of drilling fluids and cement slurries.
It is known in the art that hydroxypropyl acrylate-acrylic acid copolymer, sulfonated vinyl toluene-maleic anhydride copolymer, sulfonated vinyl toluene-maleic acid copolymer, sulfonated methyl, ethyl, or phenyl polyacrylamide, and copolymers of ethylenic acids, acrylamides and ethylenic esters of phosphoric acid, and the sodium salts thereof act as dispersants or deflocculants in drilling fluids.
Dispersant compositions typically exhibit varying properties and performance under different conditions. The performance of dispersant compositions may be evaluated in applications such as, drilling fluid dispersion, mud-to-cement dispersion, cement dispersion, low salt-low hardness mud dispersion, high salt-high hardness mud dispersion and at high temperature in all of the preceding applications.
The present invention has been developed with a view to providing improved dispersant compositions that control rheological properties of, enhance filtrate control in, and disperse one or more of spacer fluids, cement slurries and mixtures of drilling fluids and cement slurries.